Process of bleaching paper with peracetic acid



Sept. 16, 1969 M.WAYMAN ETAL 3,467,575

PROCESS OF BLEACHING PAPER WITH PERACETIC ACID Filed Aug. 2, 1965 9 Sheets-Sheet 1 BEF'ORE AGEING AFTER AGEING G. E. BRIGHTNESS, /0 0'? METABISULPHITE AFTERTREATMENT 0 S0 AFTERTREATMENT WATER AFTERTREATMENT PERACE TIC ACID ON PAPER FIG 1 Moms U410! w/M H- 09km INVENTORS Sept. 16, 1969 M. WAYMAN ETA!- 3,467,575

PROCESS OF BLEACHING PAPER WITH PERACETIC ACID Filed Aug. 2, 1965 v 9 Sheets-Sheet 2 66 o EEF0RE AGEING 0 NO AFTERTREATMENT 58 rJ S0 AF'TERTREATMENT AFTER AGEING G. E. BRIGHTNESS, l

pH of- BUFFERED PERACETIC ACID SOLUTION Mum him-m (Um-mm M 131600 1N VEN 'I'URS BY M4 1 ##mey Sept. 16, 1969 PROCESS OF BLEACHING PAPER WITH PERACETIC ACID Filed Aug. 2, 1965 GE. BRlGHTNESSfi/o M. WAYMAN ET AL 3,467,575

9 Sheets-Sheet 3 A o NEWSPRINT, BASIS WT. 32 LB.

0 ROTONEWS, BASIS WT 34. LB.

A DIRECTORY, BASIS WT. 22'5LB.

l I l l L 1 1o 20 3o 40 50 6O SECOND IN PRESS AT 1ooc FIG. 3

Mums ldavm ujrumm 5- BMW" I NVENTURS BY M f ,Qfarxe Sept. 16, 1969 M. WAYMAN 3,467,575

PROCESS OF BLEACHING PAPER WITH PERACETIC ACID Filed Aug. 2, 1965 9 Sheets-Sheet 4 I I I I o NEWSPRINT D ROTONEWS 2.0 A DIRECTORY TWO-SIDEDNESS (BRIGHTNESS DIFFERENCE BETWEEN THE TWO SIDES OF THE SHEET) O l l l l l l SECONDS IN PRESS AT 100C FIG.4

F818 WMMI BY MQ Sept. 16, 1969 Filed Aug. 2, 1965 GE. BRIG HTNESS M.WAYMAN ETAL 3,467,575

PROCESS OF BLEACHING PAPER WITH PERACE'IIC ACID 9 Sheets-Sheet 5 72 I I I I I I I 68 METABISULPHITE AFTER o PERACETIC ACID m METABISULPHITE ALONE 58 5 I l I I I l I SODIUM METABISULPHITE- G. F. L.

FIG. 5

Mam 144mm 601mm H- 303w IN VliN 'TORS BY MQ 41% may Sept. 16, 1969 WAYMAN ETAL 3,467,575

PROCESS OF BLEACHING PAPER WITH PERACETIC ACID Filed Aug. 2, 1965 9 Sheets-Sheet 6 \o 0 BEFORE AGElNG m U) n 0 Lu 2 o i- I 9 CC LU w AFTER AGEING OZINC HYDROSULPHITE SODl UM HYDROSULPHITE L 1 l l lo HYDROSULPHITE ON PAPER FIG. 6

07mm WM 4114" I]. emf! INVEN'IORS BYMQ Sept. 16, 1969 M. WAYMAN ETAL 3,467,575

PROCESS OF BLBACHING PAPER WITH PERACETIC ACID Filed Aug. 2, 1965 9 Sheets-Sheet 7 I I I l r l T BEFORE AGEING m 65 u) LLI 2 F- I 9 g AFTER AGEING c1 I| l 0 O1 MKH PO BUFFER o 0-2 MKH P0 BUFFER I l I I l l l I pH of ZINC HYDROSULPHITE BLEACHING FIG.7

97mm k/www (drum: II. 6pm

INVEN'I'URS Sept. 16, 1969 M. WAYMAN ETAL 3,467,575

PROCESS OF BLEACHING PAPER WITH PBRACETIC ACID Filed Aug. 2, 1965 9 Sheets-Sheet 8 BEFORE AGEING AFTER AGEING l l l l l I O 20 4O 6O 80 100 120 SECOND IN PRESS AT 100C FIG. 8

mm (Jam/av aim/0n M Bmwv INVIJN'IORS Sept. 16, 1969 M. WAYMAN ETAI- 3,467,575

PROCESS OF BLEACHING PAPER WITH PERACETIC ACID Filed Aug. 2, 1965 9 Sheets-Sheat 9 AFTER AGEING ORIGINAL PAPER BRIGHTNESS BEFORE AGEING 57'6 AFTER AGEING 547 V PERACETIC ACID, NO AFTERTREATMENT G. E. BRIG HTNESS, lo

D PERACETIC ACID, FOLLOWED BY 60 METABISULPHITE A PERACETIC ACID FOLLOWED BY ZINC HYDROSULPI-IITE 0 PERACETIC ACID, FOLLOWED BY METAEIISULPHITE FOLLOWED BY 55 ZINC HYDROSULPHITE l l I I l l /o HYDROSULFHITE ON PAPER 9 mm'zflwm INVLNI'ORS BY Zflr fil-ffarney United States Patent 3,467,575 PROCESS OF BLEACHING PAPER WITH PERACETIC ACID Morris Wayman and William H. Rapson, Toronto, On-

tario, Canada, assignors, by mesne assignments, to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed Aug. 2, 1965, Ser. No. 476,452 The portion of the term of the patent subsequent to Oct. 1, 1985, has been disclaimed Claims priority, application Great Britain, Aug. 13, 1964, 33,090/64 Int. Cl. D21h 3/80; D06l 3/00; D21c 9/16 U.S. Cl. 162-78 4 Claims ABSTRACT OF THE DISCLOSURE There are provided novel, economical processes for the bleaching of paper wherein bleached papers with excellent brightness and color reversion properties are obtained. In one of these processes, the paper is treated with up to about 2 percent of hydrosulphite (by weight) and then it is heated. In another of these processes, the paper is treated with from about 0.1 to about 2.6 percent of peracetic acid (by weight), heated to accelerate the bleaching reaction, and treated to neutralize the peracetic acid on the paper. In yet another of these processes the paper is treated with from about 0.1 to about 2.6 percent of peracetic acid, heated, treated with up to about 2 percent of hydrosulphite, and heated again.

This invention relates to the bleaching of paper.

In the manufacture of paper an important step is the bleaching of the fibres which will constitute the finished product. As far as we are aware, this operation is always carried out on the wood pulp. It requires mixing a pulp of high consistency with several liquids in succession, and holding the mixture of pulp and liquid in each case for several hours. A very large plant is required for these operations.

It is common practice in the manufacture of textiles to bleach the fabric rather than the unwoven fibres. The physical properties of the fabric are such that it will withstand the handling involved in impregnating and agitating it with successive bleaching and washing media. Such treatment would be impossible with paper, and no commercial process for bleaching paper has come into use in spite of the fact that a process for bleaching paper is described in Canadian Patent No. 461,242 to R. L. McEwan and F. R. Sheldon issued Nov. 22, 1949.

The bleaching of papermaking fibres as pulp is a well developed art which began when the use of rags in papermaking became widespread and which continued when the fibre base of paper changed to wood pulp.

For the bleaching of wood pulp for conversion into paper a very large plant is required, as mentioned hereinbefore. A typical modern pulp mill bleachery occupies 1,000,000 cubic feet of building space, and, with its contained equipment, may cost in the order of $6,000,- 000.00. Several operations are required during each shift, and the time and attention of foremen, supervisors, chemists, quality control testers, engineers, maintenance men, research scientists, etc., are required to bleach wood pulp in a modern mill. The typical cost of bleaching chemicals is $12.00 to $16.00 per ton of pulp, and the total cost of bleaching pulp including capital expenditure, labour, power and supplies may amount to more than twenty percent of the selling price of the product.

It is evident that a process for bleaching paper which would avoid the necessity of a large bleachery and the aforementioned costs would represent an important advance in the industry.

In accordance with this invention, we have discovered certain processes, which are set out in detail hereinafter, by the practice of which paper can be bleached, thus eliminating the need for a bleachery for bleaching wood pulp fibres prior to their formation into paper.

Pulp sheets intended for later conversion into paper regularly are bleached by spraying with peroxide, and paper is regularly whitened by adding fillers of high brightness, or white or fluorescent dyes, or by applying coatings of higher brightness than the base stock.

It should be noted that our invention does not rely on the use of fillers, dyes or coatings to increase the brightness of paper. On the contrary, our processes involve the bleaching of paper, and by the term bleaching we mean a step which effects a chemical and physical change in paper affecting the fibres making up the paper and resulting in higher brightness. The term brightness as used herein refers to the reflectivity of paper measured with a Zeiss, Elrepho (trademarks) brightness meter using a 457 millimicron filter and standardized against magnesium oxide. This measurement is the standard adopted by the Scandinavian Paper Engineers Association, and it is used in many pulp and paper mills and laboratories in North America, on the European Continent and elsewhere throughout the world, and corresponds to the TAPPI standard G.E. brightness.

As aforementioned, Canadian Patent No. 461,242 discloses a method of bleaching paper with peracetic acid. In this patent there is described a bleaching procedure in which an aqueous solution containing peracetic acid is applied to a web or sheet, and it is indicated that a brightness increment of 7.0 units is obtained, even though less than 1 percent of peracetic acid is used. This represents a significant and economical improvement. To the best of our knowledge, however, this process was never adopted in any commercial installation, and therefore has fallen short of achieving the highly desirable object of a commercial process for bleaching paper.

We have found that when paper is bleached in accordance with the process disclosed in this patent, the brightness increment claimed is obtained, but the colour of the bleached paper is not stable. Upon subjection of the treated paper to the standard heat-aging, brightness stability test, consisting of heating the paper in a circulated air oven at C. for 18 hours, the brightness deteriorated, and, indeed, the bleached paper was found to have a poorer brightness than the unbleached paper after both had been aged.

In accordance with our invention, we have discovered processes for bleaching paper by virtue of which bleached papers of good brightness stability may be obtained.

It is a well-known fact that the two-sidedness of paper, meaning the difference in brightness between the felt side and the wire side of paper, particularly newsprint, has been a serious problem in the industry.

In accordance with this invention, we have discovered a process which not only offers the foregoing advantages, but also by the practice of which the problem of twosidedness can be very materially reduced.

The processes which we have discovered for bleaching paper can be regarding in three groups: the first group involving the use of peracetic acid, the second group involving the use of hydrosulphites, and the third group involving the use of both peracetic acid and hydrosulphites. These processes will be described hereinafter with reference to the appended drawings in which:

FIGURE 1 is a graph depicting the effect on brightness, before and after aging, of neutralising the bleaching agent with various neutralising agents after the peracetic acid bleaching of newsprint and after heating of the bleached newsprint, and the effect of varying amounts of peracetic acid picked up by the paper,

FIGURE 2 is a graph showing the effect of pH on the brightness of newsprint bleached with peracetic acid before and after aging and with and without after treatments,

FIGURE 3 is a graph illustrating the effect on brightness of the duration of the heating period when different papers bleached with peracetic acid were heated after bleaching and were subsequently neutralised with an S solution,

FIGURE 4 is a graph indicating the effect on twosidedness of the duration of the heating period when different papers bleached with peracetic acid were heated after bleaching and were subsequently neutralised with an S0 solution,

FIGURE 5 is a graph showing the effect on brightness of metabisulphite concentration in a solution used to neutralise the bleach in newsprint bleached with peracetic acid,

FIGURE 6 is a graph illustrating the effect on brightness, before and after aging, of varying amounts of different hydrosulphites picked up by newsprint in processes involving the bleaching of newsprint by hydrosulphites,

FIGURE 7 is a graph showing the effect on brightness, before and after aging, of pH and buffer concentration in processes involving the bleaching of newsprint by zinc hydrosulphite,

FIGURE 8 is a graph indicating the effect on bright ness, before and after aging, of the duration of a heating period which follows the zinc hydrosulphite bleaching of newsprint, and

FIGURE 9 is a graph indicating the effect on brightness, before and after aging, of a number of different conditions in the bleaching of newsprint with peracetic acid followed by zinc hydrosulphite.

General Each of the processes to be described in detail hereinafter involves the application of a bleaching agent to paper. The bleaching agent is applied from an aqueous solution of the bleaching agent, buffered if necessary. The bleaching solution may be applied in any number of ways, e.g., by dipping the paper in the bleaching solution, by running the paper continuously through the bleaching solution, by padding with applicator rolls, by spraying, etc.

Most of the processes to be described in detail hereinafter involve the application of heat to the bleached paper. The purpose of the heating step is to accelerate the bleaching reaction. The degree of acceleration of the bleaching reaction is dependent on both time and temperature. It may be desirable to employ temperatures which permit the heating period to be as short as possible, but in all cases care must be taken to ensure that the times and temperatures which are selected will not result in discolouring of the paper, this being within the province of those skilled in the art.

The heat may be applied in a number of ways, as, for example, by clamping the paper between heated plates. For continuous operation it is contemplated that dielectric heating may be employed. Generally speaking it would be undesirable to heat the bleached paper above the steam point, i.e. 100 C. at normal pressures.

An important feature of a number of the processes set out hereinafter is the neutralising of the bleaching agent on the paper. Many types of neutralising agents may be employed. Generally the neutralising agents will be applied from aqueous solutions thereof, and the mode of application may be the same as for the solutions of bleaching agents.

Where herein we refer to percentages of bleaching agent on paper or being picked up by paper, e.g., 1% peracetic acid on paper, this is a weight percentage based on the weight of the dry paper.

It should be noted that pH values given in the following examples are measured in the solution before application to the paper.

Where reference is made herein to the unqualified term newsprint, this refers to commercial newsprint obtained from a source in Eastern Canada and made of sulphite and groundwood in conventional proportions, primarily of spruce-balsam.

In the following examples sheets of paper were cut to 10 cm. x 20 cm., and 10 cm. of each sheet was dipped in distilled water in a shallow tray. The paper was then passed upwards between a pair of rubber rollers under pressure to wring out the water from the wet half of the paper. The wet half of the paper then contained about 67 percent moisture, 33 percent fibre. The dry half of the paper acted as control.

A series of shallow plastic photographic trays was used for applying bleach. Bleach solutions were made up in the trays. Insofar as possible, buffers or other chemicals were added to the water in the trays and the pH adjusted, based on experience, before the bleaching agent itself was added. The purpose of this sequence was to minimize the time between preparation and use of bleach solutions. Before use, the bleach was analysed. Most experiments were carried out at the room temperature. To apply the bleach, the sheet was weighed, the wet half of the sheet was held immersed in the bleach solution, usually for 30 seconds, then passed through the wringer to remove excess bleach. The pH and residual bleach in the solution then were increased. A thin strip of paper was cut off and coiled in a beaker of distilled water for pH measurement.

In most experiments the paper, wet with bleach, was put between the two 10 cm. x 10 cm. heated plates of a Carver press, the plates being at a controlled temperature of C.

In many of the experiments involving bleaching, the sheets, after removal from the heated press, were dipped into a second bath for an after treatment, usually with 80,, solution or sodium metabisulphite, but occasionally with water or with other chemicals. They were again passed through the wringer, air dried in the constant temperature-constant humidity room, and tested by stand ard procedures.

Peracetic acid bleaching of newsprint It has been discovered that paper may be bleached quite successfully by a process which involves the steps of applying peracetic acid to the paper, thereafter heating the bleached paper to accelerate the bleaching reaction, and subsequently neutralising the residual bleaching agent on the paper. This process is described in the following examples.

Example I Peracetic acid solutions containing 020 grams/liter (in this procedure 7.5 g.p.l. solution is equivalent to 1% bleach on paper) together with a mixed silicate-phosphate buffer containing 7.5 g.p.l. of sodium tetrapyrophosphate and 4.0 g.p.l. of sodium silicate were applied to newsprint with the pH adjusted to 8.0 in the case of the metabisulphite series and 9.0 for the S0 and water series. The treated paper was heated in a press at 100 C. for 20 seconds, this step being followed by an after treatment by virtue of which either water, an 50 solution of pH 3, or a 7.5 g.p.l. sodium metabisulphite solution at pH 4.3 was applied to the paper to neutralise the residual bleaching agent. The paper subsequently was aged in a circulated air oven at C. for 18 hours.

The results obtained are shown in FIGURE 1. The brightness of the paper increased regularly with increasing ratios of peracetic acid, such that the newsprint originally at 57 brightness was raised to 6465 brightness by 0.5% peracetic acid on paper, and to 67-69 brightness with 1% peracetic acid on paper. When S0 or water was used as an after treatment, the brightness rose slowly as the peracetic acid solution was increased but did not quite reach 70 even with 2.6% peracetic acid on the paper.

Best results were obtained by using sodium metabisulphite to ensure adequate reducing power to destroy residual bleach and to lower pH in a controlled fashion. The metabisulphite had some bleaching ability of its own, and it can be seen from FIGURE 1 that the control, shown at peracetic acid, which had exactly the same treatment as all other papers, except that the buffer solution contained no peracetic acid, had a brightness of 59.8 after treatment, whereas the untreated paper had a brightness of 57.8, the 2 point increase being attributable to the sodium metabisulphite. When S0 was used it contributed about 1 unit brightness increase to paper not treated with peracetic acid.

When sodium metabisulphite was used for the after treatment, the brightness of the paper continued to increase at higher levels of peracetic acid on the paper at a rate of about 3.5 units per 1% peracetic acid and 75 brightness was reached with 2.5% peracetic acid on the paper.

The brightness of the paper after aging was retained quite well with both Water and S0 after treatments up to about 1% peracetic acid on the paper, but fell oif at higher ratios, whereas, when sodium metabisulphite was used as the after treatment, the aged brightness remained satisfactory at higher percentages of peracetic acid on the paper. The water treatment was less eifective than 30 treatment in controlling colour reversion on aging. The RC. numbers are given in Table I where the RC. number is the post colour number.

In the above formula, B=brightness and Ba=brightness after aging.

Water series:

TABLE I Brightness Peracetic acid (percent on paper) Before aging After agmg P.C. N0.

0 untreated 57. 8 54. 3 3. 82 0 control 59. 8 55. 1 4. 78 Metabisulphite series:

0. 64. 2 58.4 4. 84 65. 59. 7 4. 51 65. 4 59. 5 4. 63 68. 2 62. 0 4. 24 69. 0 62. 4 4. 36 71. 6 63. 2 5. 09 72. 2 63. 8 4. 92 72. 7 64. 1 4. 92 75. 0 65. 7 4. 78 S0 series:

0 control 58. 9 56. 2 0. 60. 2 56. 8 0. 61. 3 58. 3 0. 62. 9 58. 9 0. 63. 2 59. 9 0. 64. 2 59. 9 0. 65.9 61. 3 0. 66. 4 61. 6 1. 66. 6 60. 8 1. 68. 2 63. 4 1. 67. 1 60. 3 1. 68. 4 61. 6 1. 68. 7 61. 1 2.62 69. 4 59. 2

It can be seen from Table I that the brightness stability, as measured by the RC. number, remains about the same as for untreated paper and is in a range which would be considered acceptable.

6 Example 11 In this example, which demonstrates the effect of the pH of the bleaching solution on brightness, newsprint was treated with an 8.5 g.p.l. peracetic acid solution (1.2% on the paper) at varying pH in a buffer containing 4.0 g.p.l. sodium silicate and 7.5 g.p.l. sodium tetrapyrophosphate. In some cases the bleach was not neutralised after heating, whereas in other cases an after treatment using an S0 solution of pH 3 was carried out. The treated paper was heated for 20 seconds between plates at C., and aging conditions were at C. for 18 hours.

The results are shown in FIGURE 2. It can be seen from FIGURE 2 that there was no marked trend in the brightening eifect attributable to pH, although a slight maximum occurs between pHs 8 and 9. However, as shown in FIGURE 2, brightness stability was sharply affected by the pH of the peracetic acid treatment. Best results were obtained with a pH of 8.

Example III In this example, which demonstrates the effect of the duration of the heating period, 1.2% peracetic acid on the paper was obtained from an aqueous peracetic acid solution containing a mixed buffer consisting of 7.5 g.p.l. of sodium tetrapyrophosphate and 4.0 g.p.l. of sodium silicate, the pH of the solution being 9.0. The treated paper, newsprint, Rotonews and Directory, was heated for varying intervals between plates at 100 C. and an after treatment with S0 solution at pH 3 was employed. The Rotonews contained 27% sulphite, while the Directory paper was 43% sulphite, the remainder in both cases being groundwood with a small percentage of filler.

The results are shown in FIGURE 3. Examination of FIGURE 3 reveals a slight maximum after 15 seconds heating, with best results being obtained for heating period of 15-20 seconds between plates at 100 C. It should be noted that the brightness values indicated in FIGURE 3 are the average of the two sides of the paper sheet. Best results were obtained with Directory paper, Rotonews being second and newsprint third.

Example IV In this example, which demonstrates the effect of the process on the two sidedness of newsprint, the same bleached papers as used in Example 3 were employed.

It will be seen from FIGURE 4, where the results of this process are shown, that a sharp reduction in the two sidedness of the newsprint was obtained, best results occurring for papers heated between 10 and 30 seconds. The improvement in the newsprint was most marked. For untreated newsprint the difference in brightness of the two sides was about 2.4 units, and in the best case, this was reduced to 0.2 unit after about 10 seconds heating.

Example V In this example, which demonstrates the effect of metabisulphite concentration on brightness, the conditions were as follows: 1.1% peracetic acid on newsprint obtained from an aqueous solution of peracetic acid containing 4 g.p.l. sodium silicate and 7.5 g.p.l. sodium tetrapyrophosphate buffer at pH 8.0, heating for 20 seconds between plates at 100 C. followed by metabisulphite treatment at varying concentrations.

The results obtained are shown in FIGURE 5. It will be seen that the bleached newsprint brightness obtained increased with increasing concentration of sodium metabisulphite up to about 8 g.p.l. Where brightness levelled 011?. In one case the paper was not treated with peracetic acid, and the lower curve in FIGURE 5 was obtained, which indicates that sodium metabisulphite has some bleaching action by itself. The increase in brightness in this case was about two units, the original brightness of 57.4 being raised to 59.9 by 8 g.p.l. sodium metabisulphite as the only treatment. In the case where peracetic 7 acid was employed, the increase in brightness was about 4 units.

SUMMARY From the foregoing it can be seen that the best conditions for bleaching newsprint with peracetic acid were found in these experiments to be:

(a) Treatment of the moist paper with a peracetic acid solution buffered to pH 8 to 9, the buffer being made up of sodium silicate and sodium tetrapyrophosphate, followed by (b) Heating in a press at 100 C. for 15 to 20 seconds, and then (c) An after treatment with sodium rnetabisulphite solution.

Under these conditions, newsprint of 57 brightness was raised to 65 brightness with 0.25 percent peracetic acid on paper, 70 brightness with 1.2 percent peracetic acid on paper, and 75 brightness with 2.5 percent peracetic acid on paper. The bleached paper had an acceptable P.C. value under the severe aging conditions used in the test. Under the best conditions of bleaching, two sidedness was reduced and almost eliminated.

HYDROSULPHITE BLEACHING OF NEWSPRINT It has been discovered that paper may be bleached quite successfully by a process which involves the application of aqueous hydrosulphite solutions to the paper. If desired, the treated paper may be heated to accelerate the bleaching reaction but, in contrast to peracetic acid bleaching, no neutralisation of the residual hydrosulphite bleach is required. Since heating is not necessary in this case, the paper may be air dried if desirable.

It is known to bleach pulp with hydrosulphite, sometimes in conjunction with a pretreatment with H and S0 but, owing to the instability of the hydrosulphite, care always has been taken to keep it away from air. It is surprising that hydrosulphite will operate satisfactorily in a treatment of paper in which it is fully exposed to air, and that a treatment with this material alone is suflicient to produce good, comparatively stable brightness.

The following examples illustrate this process:

Example VI Solutions of sodium or zinc hydrosulphite containing 0.1 M KH PO buffered at a pH of 6.0 were applied to newsprint with the amounts of hydrosulphite picked up by the paper being varied. The treated paper was heated between plates at 100 C. for 30 seconds. Aging was at 105 C. for 18 hours.

From the results shown in FIGURE 6, it can be seen that hydrosulphites are excellent bleaching agents for newsprint, brightness being raised from 57.7 to 65 by 0.4% of either sodium or zinc hydrosulphite on paper, with good brightness retention on aging. Higher ratios of zinc hydrosulphite resulted in higher brightness, 2.0 percent on paper resulting in 69 brightness. However, treated newsprint of brightness above 65 was not as stable to the aging test. Results with sodium hydrosulphite were somewhat erratic.

Example VII In this example g.p.l. zinc hydrosulphite solutions (equivalent to 1.4% hydrosulphite on paper) at variable pHs and containing 0.1 or 0.2 M. KH PO were applied to the newsprint. The treated papers were heated for 20 seconds between plates at 100 C. Aging conditions were as in Example 6.

The results are shown in FIGURE 7 which indicates that brightness was best at pH 4.0 to 5.0, that brightness stability was best at pH 5.0, and that buffer concentration has a significant effect on brightness, the more concentrated buffer resulting in 1.5 units higher brightness.

Example VIII Example 7 was repeated using hydrosulphite solutions of pH 5.0 and containing 0.1 M KH PO The treated 8 paper was heated for varying intervals between plates at C.

The results obtained are shown in FIGURE 8. As may be seen by reference to this figure, best results were ob tained with very short heating times of the order of 5 seconds. Heating for periods longer than 30 seconds resulted in losses in both brightness and brightness stability to aging.

The best conditions for bleaching newsprint with zinc hydrosulphite were found in these experiments to be:

(a) Treatment of the moist paper with zinc hydrosulphite solution buffered to pH 5.0, the buffer being 0.2 M KH PO followed by (b) Heating in a press at 100 C. for 5 to 30 seconds.

Newsprint of 57.5 brightness was raised to 65 brightness by bleaching with 0.4 percent zinc hydrosulphite on paper, and 69 brightness with 1.4 percent zinc hydrosulphite on paper. Sodium hydrosulphite was as effective as the zinc compound in raising the brightness of the newsprint to 65, but the results on addition of higher ratios of sodium hydrosulphite were erratic.

Multi stage bleaching of newsprint It has been discovered that paper may be bleached quite successfully by a process which involves the steps of applying peracetic acid to the paper, heating the treated paper, followed by the application of a hydrosulphite, or by such a process where sodium metabisulphite is applied to the paper after treatment with peracetic acid and heating but before treatment with the hydrosulphite. Heating is not required after the hydrosulphite treatment and the paper may be air dried if desired. Heating of the paper after the hydrosulphite treatment is desirable, however, as is evidenced by FIGURE 8.

The preferred conditions for carrying out these multistage bleaching processes are as previously indicated in connection with the individual processes described in the previous examples.

It is known to bleach pulp using in sequence an alkali metal peroxygen compound, an acid sulphite or S0 solution and a hydrosulphite, but the conditions of operation in the bleaching of pulp on the one hand and paper on the other are completely different, and for the aforementioned reasons, it would not have been expected that hydrosulphite could be used successfully for the latter.

The following example is illustrative of these multistage processes:

Example IX Newsprint was treated with an aqueous solution of peracetic acid buffered with 4 g.p.l. sodium silicate and 7.5 g.p.l. sodium tetrapyrophosphate at pH 8.0. The pick up was 1% peracetic acid on to the weight of the paper. The treated paper was heated in a press at 100 C. for 20 seconds. In one case no after treatment was employed. In another case the paper was treated with a 7.5 g.p.l. so dium metabisulphite solution. In still another case the paper was treated with zinc hydrosulphite solutions buffered to pH 5.0 with 0.1 M KH PO In still another case the paper was treated with both the sodium rnetabisulphite and zinc hydrosulphite solutions mentioned hereinbefore with the sodium metabisulphite treatment preceding the zinc hydrosulphite treatment. In the latter three cases the paper was heated in a press for 20 seconds at 100 C. In all cases aging was as in Example 8.

The results are shown in FIGURE 9.

It is evident from FIGURE 9 that the two stage bleach resulted in high brightness, the newsprint being raised from 57.6 to 75.3 by the use of 1.0 percent peracetic acid following by 1.2 percent zinc hydrosulphite on paper. The brightness stability to aging at C. for 18 hours was satisfactory, P.C. numbers being in the range of 4.2 to 7.1.

At very low applications of zinc hydrosulphite, up to 0.2 percent on paper, the use of a sodium metabisulphite intermediate stage was advantageous, and indeed the small bleached grade, the best brightness obtained being 36.2. amount of zinc hydrosulphite did not improve the bright- However, the laboratory papers containing 20 percent unness or brightness stability over the metabisulphite alone. bleached kraft and 80 percent groundwood were raised This was in the brightness range up to 70', and this obto an acceptable newsprint brightness level by either servation could be of some importance for newsprint peracetic acid or by zinc hydrosulphite when the groundbleaching. At high brightness levels, or high ratios of wood was spruce, or by the two stage process when hemzinc hydrosulphite to paper, the sodium metabisulphite lock groundwood was used.

was not advantageous, and in fact resulted in somewhat The refiner groundwood paper was brightened more lower brightness than the direct two stage peracetic acidby zinc hydrosulphite treatment than by peracetic acid zinc hydrosulphite procedure. treatment. The two stage process however, raised the Comparison with FIGURE 1 shows that 2.5 percent brightness of this paper about 12 units.

peracetic acid followed by 1.0 percent sodium meta- The pine high yield sulphite paper was of very poor bisulphite was as effective as the best of these results in brightness (37.8) as received, and any of the processes raising the brightness of the same newsprint, but at presimproved it, but even the two stage Process y raised cut the costs of chemicals are such that the two stage h brightness to h corresponding Pep made process with hydrosulphite would be advantageous. Hyof Spruce began at bnghtnes? was ralseq by the drosulphite alone, as shown in FIGURE 6 did not achieve two Stage process to f f acld cons}derably the e high level of brightness more eifective than hydrosulphlte in bleaching this paper.

In addition to newsprint, thirteen other types of paper gg g gg i e g g fe fi glli igg l gs: ig were bleached bylthe procedures l t herem' The 74.6 brightness from 61.5 by the two stage process. The papers were most y greendweodeentalemg papers but laboratory paper, of which spruce groundwood formed 75 Pnbleaehed kraft and hlgh W sulphlte Papers were percent and high yield spruce sulphite the remainder, bemcluded. Laboratory papers included newsprint made of haved as would have been expected rb on the groundvfwd and Pnblfiached kraft Pulp PlaCe of P haviour of the two components, whereas the correspondconventional sem1-bleached kraft pulp. Four bleaching i i Paper responded better h expected to 11 three Processes were used, peracetic acid, PefOXide, Zinc y processes, and particularly to the zinc hydrosulphite treatsulphite and the two-stage peracetic acid zinc hydroment and its brightness was raised by the two stage process sulphite process. The results are shown in Table II. from 53.4 to 70.1.

TABLE II.PAPER BLEACHING See legend following table for the meaning of the symbols. (a) Paper Description.

No. Description 1-Newsprint, sulphite, groundwood 9-Laboratory news 10 percent W.C krait, 90 percent refiner g.w. 2--Rotonews A, sulphite, goundwood 10Sulphite, 68 percent yield, pine. 3-Rotonews B, 27 percent sulphite, 73 percent groundwood 11-Laboratory paper, 25 percent pine H.Y. sulphite, 75 percent spruce 4--Directory, 44 percent sulphite, 56 percent groundwood g.w. 5Krait, West Coast coniferous. 12-Spruce g.w. 6-Laboratory news" 20 percent W.C. kraft, 80 percent hemlock g.w. 13Sulphite, 67 percent yield, spruce. 7-Laboratory news 20 percent W.C. kratt, 80 percent spruce g.w. 14Laboratory paper, 25 percent spruce H.Y. sulphite, 75 percent spruce 8Refiner g.w. West Coast coniferous. g.w.

(b) Brightness after bleaching.

Untreated Peracetie acid Zinc Hydrosulfite Two stage P.A.Z.H. Paper No. B Ba P C B Ba P O G B Ba P C G B Ba P C G 57. 6 54. 7 3. 15 69. 0 63. 1 3. 83 ll. 4 67. 5 60.7 4. 90 9. 9 72.8 66. 0 3. 68 15. 2 60. 2 55. 2 5. 0 70. 5 64. 0 3. 96 10. 3 69. 7 62. 0 5. 06 9. 5 72. 9 66. 8 3 3 12. 7 61. 8 59. l 2. 4 73.0 67. 2 3. 02 11.2 72. 5 65. 3 4. 00 10. 7 75. 8 69. 1 3 0 14.0 65. 0 60. 9 3. 2 74. 8 69. 1 2 66 9. 8 73. 6 66. 9 3. 8. 6 76. 7 70.7 2 6 11. 7 26. 2 26.0 36. 2 33. 7 10, 0 31. 7 30. 1 5. 5 35. 3 33.3 9. 1 46. 4 43. 9 4. 89 56.9 52. 2 7 10. 5 53. 3 49. 4 5. 45 6. 9 60. 1 55. 0 5. 2 13.7 52. 4 48.0 6. 38 60.0 56.0 3. 96 7. 6 60.6 55.2 5. 37 8. 2 63. 6 57. 8 5. 0 11.2 56.3 51. 6 5. 74 63. 7 55.9 7. 06 7. 4 66.0 55.7 8. 86 9. 7 68. 3 59. 6 6. 30 12.0 50.8 46. 8 6. 41 59. 3 53. 3 6. 49 8. 5 59.3 51. 5 8. 87 8. 5 62. 5 56.2 5. 82 11.7 37. 8 35. 2 8. 47 48. 1 44. 9 5. 81 10.3 49. 5 42. 5 13. 14 11.7 51.7 45.0 11. 05 13.9 53. 4 50. 8 3. 50 64. 5 59. 9 3. 11. 1 66. 0 59. 3 5. 21 12. 6 70. 1 62. 9 4. 50 16. 7 61. 5 59. 0 2. 20 71. 9 66. 1 3. 20 10.4 72.2 65. 2 3. 94 10. 7 74. 6 68. 6 2. 77 13. 1 50.6 48. 0 58. l 54. 3 4. 12 7. 5 56. l 49. 5 8. 58 5. 5 61. 3 55. 2 5. 97 10.7 58. 4 55. 5 3. 02 67. 8 63. 1 3. 14 9. 4 67. 5 60. 7 4. 49 9. 1 70. 9 65. 5 3. 12 12. 5

Average 9. 7 9. 1 12. 7 B=Brightness; Ba=Brightness after aging at 105 C. for 18 hours; PO=P Q number; G=Bleached brightness minus untreated Brightness Gain) Peracetic Acid. 1.1 percent peracetic acid on paper, bufiered to pH 8 0 with 4 0 g p 1 sodium silicate and 7 5 g p 1 sodium tetrapyrophosphate, heated between plates at 100 C. for 20 seconds, followed by 7.5 g.p.l. sodium metabisul hite.

P Zine Hydrosulphite: 1.2 percent zinc hydrosulphite on paper, buffered to pH 5.0 with 0.1 M KHzPOA, heated between plates at 100 C. for 20 seconds. Two stage PA-ZH: 1.1 percent peracetic acid on paper, buffered to pH 8.0 with 4.0 g.p.l. sodium silicate and 7.5 g.p.l. sodium tetrapyrophosphate, heated between plates at 100 C. for 20 seconds, followed by 1.2 percent zinc hydrosulphite on paper, buffered to pH 5.0 with 0.1 M KHzPOl. heated between plates at 100 C. for 20 seconds.

The average brightness gain for this variety of papers 65 Examination of the P.C. numbers shows that these bleached by these three processes are shown atthe bottom processes did not, for the most part, adversely affect the of the G columns, the two stage peracetic acid-zinc hydrobrightness stability to aging of any of these papers, imsulphite process resulting in an average brightness gain proving it in many cases.

of 12.7 units, peracetic and metabisulphite 9.7 and zinc Aside from brightness other properties of newsprint hydrosulphite 9.1. The process conditions were reasonably bleached by processes within the scope of this invention close to optimal for each process, as shown by the previous were examined. The following Table III indicated the results for newsprint, but not necessarily optimal for each results of colour tests made on newsprint bleached with variety of paper. 0.8 percent peracetic acid on paper (pH 9.0) followed The unbleached kraft paper, paper No. 5 was brightby heating and an after treatment with an S0 solution ened most by peracetic acid, but was still below a semiat pH 3.

TABLE IIL-OPTICAL PROPERTIES OF BLEACHED N EWSPRINT Untreated Bleached Curve N o 6725 6726 Reflectance at 457% millimicrons. 0. 556 0. 663 Reflectance at 570 millimicrons 0.678 0.819 X 0. 33% 0. 3326 Y 0. 3410 0. 3423 Visual efliclency 0. 670 0. 804 Dominant wavelength. 576. 4 675. 6 Percent saturation 12. 7 12. 9 Scattering ooeflicient (s) 0. 093 0. 096 Absorption coefficient (k). 0. 0074 0. 0023 Printing opacity (actual) 0. 954 0.900 Printing opacity (32 lb.) 0. 949 0. 889 Basis wt 33.2 34. 3

It will be seen that printing opacity has been somewhat reduced by the process, while the percent saturation is hardly changed, and while the dominant wavelength trends towards the blue, it does so to a less extent than normal than when pulps are bleached.

It is contemplated that the processes hereinbefore described may be carried out directly on a paper machine with the bleaching solution being applied at the wet end of the paper machine, the heat to accelerate the bleaching reaction being obtained in the dryer section and the neutralising solution, where required, being applied at some point between the beginning and end of the dryer section.

What we claim as our invention is:

1. A process for bleaching paper, which comprises:

(a) applying to the paper from about 0.1 to about 2.6

percent of peracetic acid (by weight);

(b) heating the treated paper at a temperature and for a time sufficient to accelerate the bleaching reaction between the peracetic acid and the paper, while being insufficient to discolor the paper; and

(c) neutralizing the peracetic acid on the paper by application thereto of a compound selected from the group consisting of sodium metabisulphite and sulfur dioxide.

2. The process of claim 1, wherein:

(a) the peracetic acid is in a buffered solution at a pH of from about 8 to about 9; and

(b) the treated paper is heated at a temperature of about C. for from about 15 to about 20 seconds.

3. The process of claim 1, wherein the peracetic acid is in a buffered solution at a pH of about 8 to about 9, which buffered solution comprises sodium silicate and sodium tetrapyrophosphate.

4. The process of claim 2 wherein the buffered solution of peracetic acid comprises sodium silicate and sodium tetrapyrophosphate.

References Cited UNITED STATES PATENTS 3,085,039 4/1963 Gartner 162-78 1,154,851 9/1915 Foley 162-13 1,645,061 10/1927 Kress 162-85 2,125,634 8/1938 Heritage 162-13 2,150,926 3/1939 Kaufimann et a1 162-13 2,347,434 4/ 1944 Reichert et a1 162-78 X 2,492,047 12/1949 Burg et a1 162-17 2,598,580 5/1952 McEwen et a1 162-13 2,862,784 12/1958 Kise et a1. 162-78 2,963,395 12/1960 Back et a1 162-83 2,970,882 2/ 1961 Kumin et a1. 8-156 X FOREIGN PATENTS 461,242 11/1949 Canada.

S. LEON BASHORE, Primary Examiner US. Cl. X.R. 8-111; 162-84 

